Normal internet applications are great for letting users talk to servers (or other users for that matter), but not so good when one of your scripts wants to ask a remote site for some data. Enter web services - a handy interface to remote data stores that can be queried to narrow down to just what you need. PHP has several ways to go about using (and creating) these, but one of the most popular, moreso for PHP4 users, is NuSOAP. Using it can be a little tricky, though, so thankfully there's tutorials like this on DevBuilder to get you started.
In this article we will first try to define Web Services, their advantages and very basic architecture. Then we will walk through two examples, showing how to create Web Services with the PHP NuSoap toolkit and how to invoke that service, which was developed in Visual Basic 6.0. In the second example we show how to use Web Services with PHP. These examples demonstrate interoperability of Web Services (platform, operating systems and language independent).
They provide a great overview of what web services are/how they're used/benefits before even getting into the technical side of things. From there, it's all examples and screenshots of their example to grab information from a remote service - GeoCash from serviceobjects.net.